The use of investment casting to produce metal objects with detailed interior structures has grown over the years to include many novel structures and now is used in a wide variety of industrial casting jobs. Complicated components such as turbine blades with interior passages, and cellular solids, have produced a need for investment materials and process that will permit the accurate formation of such parts while facilitating the removal of all investment material residue adhering to the casting.
In the investment casting process an expendable pattern is covered with a slurry suspension of an investment material that subsequently hardens to form a mold. After the investment material has solidified the pattern is removed without disturbing the mold, typically through the application of intense heat that vaporizes the pattern in a process known as the ‘burn out’. The resulting, highly detailed, high fidelity, mold cavity is filled with molten metal to create a casting in the shape of the original pattern. When the metal has solidified the casting is freed from the mold by breaking up or dissolving, and thus destroying, the mold.
The general properties for investment materials and the investment casting process are well described for dental, and similarly for jewelry, casting in the standard reference: “Philips' Science of Dental Materials” chapter 22. Bidwell, gives a broad survey of industrial investment materials and processes “Investment Casting”, chapters 3 and 5.
Investment materials consist primarily of two ingredients: a refractory and a binder. The majority of the investment material is usually comprised of the refractory, which serves to withstand the intense head of the molten metal during the metal casting procedure. The binder serves to glue the refractory together to create a hard, strong mold that can withstand the weight and force of the molten metal entering the mold. A liquid, usually water, is mixed with the dry refractory and binder to create a slurry of the required density and viscosity for the casting process.
Investment materials have been in use since ancient times and numerous ingredients have been used in their formulations. Modern investment casting processes typically use various types of powered silica as the refractory material with gypsum, phosphate compounds, or silicate compounds as the binder. The liquid solvent is either water or alcohol based, and may contain dissolved agents to aid in the binding process. The gypsum and phosphate binders are primarily used in the dental and jewelry fields, while the silicate binders find greatest use in industrial casting processes.
Numerous properties of investment materials are important to the investment casting process, with some properties more important to specific casting applications than others. Among these properties are high temperature resistance, dimensional stability and change during casting process, strength, and gas permeability or porosity.
The issue of mold strength has been a major focus in the development of investment materials, with the principle aim of increasing the compressive strength of these materials. It is believed that increased strength will produce more accurate molds that will resist cracking or deformation during the investment casting process. To this end various forms of gypsum have been developed for dental and jewelry applications with progressively higher strength, as indicated in Philips'. A lower proportion of water in the investment material mixtures has also been found to increase the strength of investment molds, as is also illustrated in Philips'. Most modern investment mixtures are of the high strength variety.
The investment casting process not only involves the forming of the mold and the casting of the metal, but also requires the removal of the mold after the casting has cooled and sufficiently solidified. Typically, this involves a mechanical “knock-out” of the brittle investment material, either by manually striking the mold with hammering tools, or the use of vibrating equipment, or both. In some cases investment material tenaciously adheres to the casting, or is positioned in inaccessible places on the casting, so alternative means must be used to fully remove all traces of material. These additional methods include mechanical shot blasting, ultrasonic vibration, and chemical means.
The chemical means of removing investment material principally makes use of hydrofluoric acid, as in U.S. Pat. No. 4,025,361, a highly toxic chemical that dissolves the silica refractory component of the investment material. Acids similar to hydroflouric such as flouroboric claimed in U.S. Pat. No. 2,502,337, and flourophosphoric claimed in U.S. Pat. No. 2,666,001 that are also high toxic, corrosive, and environmentally hazardous. Other traditional chemical means make use of strong caustic solutions under heat and pressure. Both the acid and caustic chemical methods are limited to use with a limited number of metals, are dangerous to use, are expensive, and environmentally harmful. A proprietary chemical method of investment material removal is available from Kolene, described in Kolene company literature, which dissolves the investment material in a molten salt but is limited in its application to a residue scavenger process due to the expense and difficulty of use.
The prior art investment removal processes that use mechanical means are limited to mechanical shock and vibration applied to the exterior of the casting that cannot directly access any interior spaces of the casting. The prior art investment removal processes that use chemical means operate by dissolving one or more of the investment material components so as to carry away the components in solution. The high strength of investment materials formulations in prior art also inhibits the removal of the mold after casting due to the low frangibility of the mold material.
Most patents related to investment casting are variations of the basic investment material formulations, and the basic investment casting processes related above, to suit a specific casting application. Numerous patents exist for dental and industrial investment materials. Dental patents such as U.S. Pat. No. 4,604,142 which claims superior qualities of mold expansion while preserving gas permeability, under the typical dental casting process, and U.S. Pat. No. 5,373,891 claims an improved gas permeability of the investment material for typical dental use only. U.S. Pat. No. 2,753,608 also claims improved gas permeability of the investment material mold made with gypsum binder, but only for high-strength molds, and principally claims a non-reactive surface property of the mold for casting easily oxidized. A standard art casting, and mold knockout process, is assumed for all of the investment materials cited in these patents. None of these patents claim any novel properties related to mold removal, or non-mechanical methods of mold removal.
U.S. Pat. No. 3,966,479 claims an investment material made with gypsum binder of lower compression strength than previous materials, but is used for dental work only. This patent assumes the typical dental casting process and knockout of mold by mechanical means and makes no claims as to mold removal in relation to the strength of the investment material or mold.
U.S. Pat. No. 3,540,519 claims an investment material formulated specifically to aid in the removal of the mold after the casting is formed. This mold material is designed to thermally fracture on cooling from the high temperatures after the casting is made. This method is limited to specific casting conditions.
Investment materials and processes for casting detailed interior structures are described in U.S. Pat. No. 3,946,039 for the casting of cellular aluminum foam. The process makes use of a gypsum-binder investment material cast using the block casting process. Several processes to remove the investment material from the interior spaces of the casting are described in this patent. The principle investment removal method is limited to the use of pressure jets of water directed at the investment material to break it into small bits that are subsequently washed away. The depth that the water jet is able to penetrate into the casting interior limits this investment removal process to small castings. Typically the water jet method is limited to removing investment material within two inches from the surface resulting in a maximum casting size in a single dimension of four inches as indicated in the ERG Duocel literature. Large cellular structure castings are not possible by any known method.